By phase alignment, I don't mean like the user-adjustable knob on today's powered subwoofers. I'm referring to the phase alignment between drivers in a full-range speaker, which is typically accomplished to some extent or not at all by today's passive crossover networks -- creating phase shift that slightly delays the tweeter's output so that wavefronts from the tweeter's acoustic center and the woofer's acoustic center reach your ears at exactly the same time. Usually, crossover designers try to place the crossover point at a frequency where the phase shift caused within the drivers themselves is about the same. Some speakers you'll see have their front baffle angled backward. This isn't done to direct the sound upward -- it's done to bring the acoustic centers of the tweeter and woofer in line. However, the high-order crossover slopes that would be possible with active circuitry would almost render phase-alignment a nonissue since frequency bands in which the tweeters and woofers overlap would be extremely small.

Addendum: I just noticed what you wrote about a coincident-source driver. Perhaps you're familiar with the work of Jim Thiel? He developed a 3" driver that integrates a 3" aluminum cone and a 1" aluminum dome on the same voice coil, with a compliant material between the voice coil edge and the cone itself creating a mechanical crossover between the two diaphragms. Below the mechanical crossover frequency, the two diaphragms move as one. But if Thiel decided to build computer speakers, I probably couldn't afford them. For us, it's probably better to look at high-quality wide range cone drivers, such as the Tang Band W3-593 and W3-871 (which are at Parts Express). I've used the W3-593, and found that it provided such excellent high frequency extension that a tweeter really wasn't needed.